Temperature dependence of aragonite and calcite skeleton formation by a scleractinian coral in low mMg/Ca seawater

Abstract

Temperature-dependent aragonite and calcite formation by scleractinian corals were examined in low molar (m) Mg/Ca seawater, the experimental conditions replicating the fluctuating mMg/Ca levels prevailing throughout the Phanerozoic Eon. Incubation and skeletal growth monitoring of juveniles of the scleractinian coral Acropora solitaryensis for 4 months from the planula stage, in seawater with mMg/Ca ratios of 5.2, 1.0, and 0.5, and temperatures of 19-28 °C, indicated that polymorphism of present-day scleractinian corals in low mMg/Ca seawater is also influenced by seawater temperature. However, corals produced more aragonite than formed in inorganic CaCO 3 precipitation experiments under the same conditions, except at 19 °C. Although the aragonite content reflected the results of the latter (abiotic) experiments at 19 °C, it is suggested that aragonitic scleractinian corals controlled skeletal formation biologically under low mMg/Ca conditions at higher temperature, growth rates being faster at 25 °C and slower at 19 °C for all mMg/Ca ratios. Compared with growth rates under the present-day-equivalent seawater Mg/Ca level of 5.2, juvenile growth decreased by 62.8% ± 14.7% and 56.7% ± 6.7% under mMg/Ca levels of 1.0 and 0.5, respectively; the results suggest that growth of aragonitic scleractinian corals is suppressed throughout varying seasonal temperatures under low mMg/ Ca conditions. This supports previous findings from variable temperature perspectives that scleractinian corals grow more slowly in low mMg/Ca (Cretaceous) seawater, interpreted as a possible explanation for the hiatus in scleractinian reef building in the Cretaceous Period.

Peer Review #1 of "Differential gene expression in skeletal organic matrix proteins of scleractinian corals associated with mixed aragonite/calcite skeletons under low mMg/Ca conditions (v0.1)"

“…5 juvenile skeletons produced in mMg/Ca 5.2 treatment and 20 juvenile skeletons produced in mMg/Ca 0.5 treatment were analyzed by X-ray diffractometer (SmartLab, Rigaku, Japan) with a low background silicon holder. Calcite intensity was much stronger than that of aragonite, the specific peak of the latter (<10 wt%) being almost equivalent to the background (as described in Higuchi et al 2017). The presence of aragonite was therefore confirmed by Meigen's stain at 85…”

“…In this study, genes involved in carbonate chemistry of the calcifying fluid, such as Ca ATPase (Zoccola et al 2004) and bicarbonate anion transporter (Zoccola et al 2015), were in neither the top 50 up-or down-regulated genes, nor the full list of DE genes. Thus, we propose that change in mMg/Ca in seawater did not impact the carbonate chemistry of the calcifying fluid, although the calcification rate decreased with low Mg/Ca seawater (Higuchi et al 2014;Higuchi et al 2017).…”

“…Although scleractinian corals usually produce aragonite skeletons, they can produce calcitearagonite mixed skeletons (referred as calcite coral) in low-Mg/Ca conditions (Ries et al 2006;Higuchi et al 2017). To understand the formation mechanisms of each crystal type in corals, the gene differential expression profile was compared between calcite and aragonite corals.…”

“…Seawater was changed every 2 -3 days. Manipulated seawater (mMg/Ca = 0.5) was prepared by mixing filtered (pore size: 0.22 µm) natural seawater and Mgfree artificial seawater, as described in Higuchi et al (2017). Four containers (each containing approximately 50 polyps) were prepared, two for natural seawater (mMg/Ca = 5.2) and the other two for low-Mg seawater (mMg/Ca = 0.5).…”

“…There is great diversity in calcite versus aragonite spicules in sponges (Uriz 2006), and calcite versus aragonite versus a mix in mollusk (Belcher et al 1996). Although each crystal type is specifically determined by each marine organism, environmental conditions, such as molar Mg/Ca ratio (mMg/Ca) and seawater temperature, can also influence crystal type selectivity (Ries et al 2006;Higuchi et al 2017). Manuscript to be reviewed (Ries 2010).…”